1. Field of the Invention
The present invention relates to a detection apparatus, a detection method, and an electron beam irradiation apparatus for detecting a signal for focusing an electron beam with which a master of an optical disk, for example, is prepared.
2. Related Art
In recent years, there has been a demand for an optical disk of increasingly higher recording density. In order to realize such a high recording density, it is necessary to form a smaller recording pit. As for preparation of a master of optical disk, earlier applications have been filed (Japanese patent application number H04-276068 and Japanese patent application number 2000-57374), in which electron beam irradiation apparatuses are proposed for recording information by irradiating a master disk with an electron beam capable of forming a finer pit than a conventional laser light.
In apparatuses for recording information by irradiating the master disk with an electron beam, grooves/lands, pits and the like having a width of a few to tens of nm may be formed. However, in order to record information with higher accuracy, it is necessary to accurately adjust an electron beam irradiation distance for suitable electron beam irradiation according to the distance between an electron beam irradiation head and the master disk.
For example, in a partial vacuum electron beam irradiation apparatus employing a differential static pressure floating pad such as the above-mentioned electron beam irradiation apparatus, positive pressure is applied to a static pressure bearing unit of the differential static pressure floating pad and negative pressure is applied so as to attract an exhaust unit, whereby the master disk and the differential static pressure floating pad are sticked in a contactless manner, keeping a gap of the order of μm so that a vacuum degree becomes higher in a position which is closer to the center of the differential static pressure floating pad. In this case, a vacuum degree that does not influence the irradiation with electron beams (approximately 1×10−4 [Pa]) may be achieved at the center of the differential static pressure floating pad.
A conventional scanning electron microscope, as shown in FIG. 10, will be described as a general electron beam irradiation system and a general electron beam detection system.
The scanning electron microscope 100 includes a so-called electron beam column 101 and an electron beam irradiation head 106 maintaining a position facing a specimen 103 with a microscopic gap so as to permit electron beam irradiation. The specimen 103 is subject to irradiation with an electron beam 102 emitted from the electron beam irradiation head 106.
The electron beam column 101 includes an electron gun 111, a condenser electron lens 112 for condensing the electron beam 102 emitted from the electron gun 111, an electron beam modulator 113, a limiting plate 115 having an aperture 114 in its center, an electron beam deflector 116, a focus adjusting electron lens 117, and an objective electron lens 118.
The electron beam modulator 113 is formed of opposing deflection electrode plates, for example, across which a predetermined voltage is applied to deflect the electron beam 102, thereby causing the electron beam 102 to pass through the aperture 114 of the limiting plate 115 and to be intercepted by the limiting plate 115 so as to achieve ON/OFF modulation.
Further, the electron beam deflector 116 is formed of opposing deflection electrode plates, for example, such that a position of irradiation of the electron beam 102 may be finely moved in a radial direction on the specimen 103 or in a transversal direction in relation to tracks.
The focus adjusting electron lens 117 and the objective electron lens 118 each include an electromagnetic coil, which renders the electron beam 102 to go through the electron beam irradiation head 106 so as to focus on the specimen 103.
The focus adjusting electron lens 117 may be manually adjusted and held in a focus state. The focus adjusting electron lens 117 may be supplied with a focus error signal from a focus control unit, for example, so as to control the focus.
In addition, the electron beam 102 employed in the irradiation of the specimen 103 is deflected by an amount that depends on the shape of the surface of the specimen 103 and constitutive material thereof, so that a corresponding portion becomes deflection electrons 107. For example, the amount of the deflection electrons 107 may be determined by a semiconductor detector 108 mounted on the electron beam irradiation head 106. It should be noted that the semiconductor detector 108 includes, for example, a plurality of ring-shaped P-N junction semiconductor devices.
Further, as the electron beam irradiation requires a vacuum environment, the scanning electron microscope 100 employs a total vacuum system in which all components, such as the electron beam column 101, the electron beam irradiation head 106, the semiconductor detector 108 and a specimen chamber in which the specimen 103 is located are accommodated under vacuum condition.
An exhaust means of a vacuum pump (not shown) is connected to the electron beam column 101 and the exhaust means sucks air of the internal portion of the electron beam column 101, so that the inside portion of the electron beam column 101 is maintained at a vacuum degree (1×10−4 [Pa]) that does not influence the irradiation of electron beams.
The thus-constructed scanning electron microscope 100 operates as follows.
The electron beam 102 is emitted from the electron gun 111 of an electron beam energizer located at an upper portion of the electron beam column 101, which is mounted above the electron beam irradiation head 106.
The electron beam 102 emitted through the electron beam irradiation head 106 and reaching the specimen 103 shows three patterns of behavior, namely incidence of electrons into the specimen 103, transmission of part of the electrons, and deflection of the remaining. In this case, the semiconductor detector 108 determines an amount of absorption current caused by the incidence of electrons and the transmission of electrons and an amount of secondary electrons or deflection electrons bounced by the specimen 103 so as to obtain an observing image signal 121, of the specimen 103, processed by a signal processing circuit 119. Accordingly, the amount of signals corresponds to an image quality of the specimen 103, so that it is necessary to increase the amount of signals in order to improve image quality.
For example, the deflection electrons originating from the specimen 103 due to the electron beam irradiation start out upwardly at a deflection angle of 45° at one side, with respect to an axis of the electron beam. Since there is no blocking component above the specimen 103, the semiconductor detector 108 may be mounted at an end of the electron beam irradiation head 106 of the electron beam column 101 so as to capture a sufficient amount of deflection electrons to be determined. By using the focus adjusting electron lens 117, a sharp observing image needed to focus the electron beam may be obtained.
FIG. 11 is a schematic diagram showing the specimen and a deflection electron detector.
In FIG. 11, the deflection electrons 107 of the incident electron beam 102 to the specimen 103 are detected by divided semiconductor detectors 108A and 108B. Detected signals A and B detected by the semiconductor detectors 108A and 108B are amplified by amplifiers 122A and 122B, added (A+B) and subtracted from (A−B) by a processing unit 123 so as to obtain a composite image 124 and a rugged image 125. The amplifiers 122A and 122B and the processing unit 123 shown in FIG. 11 correspond to the signal processing circuit 119 shown in FIG. 10.
FIG. 12 is a cross-cross-sectional view showing an arrangement of a deflection electron detector in a conventional electron beam irradiation apparatus. In the conventional electron beam irradiation apparatus shown in FIG. 12, an electron beam irradiation opening 133 of a static pressure floating pad 131 is formed in a taper expanding towards an electron beam column 132 and at an angle of 45° at one side, so that a deflection electron detector 134 mounted at an end of the electron beam column 132 may sufficiently capture deflection electrons from a master disk 135.
On the other hand, besides the conventional electron beam irradiation apparatus, as described above, the applicant of the present invention filed, for example, another Japanese patent application No. 2002-50146 “ELECTRON BEAM IRRADIATION APPARATUS AND ELECTRON BEAM IRRADIATION METHOD”. FIG. 13 is a cross-cross-sectional view showing an arrangement of a deflection electron detector in a conventional electron beam irradiation apparatus. As mentioned in the application, when replacing a master disk, it is necessary to retreat a static pressure floating pad from the master disk, which may cause several drawbacks, such as scattering of electron beams because of a decreased vacuum degree from the static pressure floating pad to an inside portion of the electron beam column. As shown in FIG. 13, a vacuum seal valve is illustrated. At an end portion of the static pressure floating pad 141, air is supplied via an air joint 147 against a resilience of a compression spring 146 so as to move a piston 145, close an electron beam irradiation opening 143, and thereby obtain a vacuum seal condition.